Generally speaking, high density printed circuit boards are constructed with several electrically conductive layers separated by dielectric layers. Some of the conductive layers are utilized to supply power and ground voltages. The remaining conductive layers are patterned for electrical signal interconnections among integrated circuit chips. Layer-to-layer interconnections are achieved by means of through-holes plated with electrically conductive material. In high density printed circuit boards it has been normal practice to provide interconnections between adjacent conducting layers, which interconnections are commonly known as "vias".
It has been the task of circuit board designers and scientists to achieve a balance between many competing and interdependent parameters in order to obtain optimum performance.
Many times parameter selection is limited or influenced by the available choice of materials and/or the processes by which these materials can be used. For example, signal lines will often pick up energy from adjacent lines, known as "signal cross-talk" or "coupled noise". Minimum spacing between adjacent lines is influenced by this signal cross-talk. A decrease in the dielectric constant (a material dependent parameter) of the dielectric sheet of the uncircuitized core is known to improve wiring density. However, as materials with lower dielectric constants are chosen, i.e., materials having a dielectric constant of 3.0 or less, it becomes increasingly difficult to process them into workable laminates. These materials are not easily bonded to surrounding or adjacent layers, nor are they easily adhered to plated through-holes.
Polytetrafluoroethylene is a prime example of a material which is difficult to laminte or adhere to other surfaces. Yet, this material has a dielectric constant of approximately 2.1, which makes it ideal as a material which would improve performance ad dimensional characteristics.
In U.S. Pat. No. 3,305,416; issued: Feb. 21, 1967, it is suggested that a dielectric sheet for use in circuit boards can comprise a polytetrafluoroethylene as one of many desirable materials. A conversion coating is necessary for placement on adjacent metal layers, before bonding to the dielectric layer can be achieved. No mention, however, is made therein of the particular difficulty in which polytetrafluoroethylene fails to adhere to the plated through-holes.
Despite twenty years of research since the issuance of this patent, no convenient or commercial means has been found to bond this low dielectric material to plated through-holes, and thus, the use of polytetrafluoroethylene for dielectric sheet has not been of practical consequence in circuit board construction.